1. Field of the Invention
The present invention relates to an optical fiber grating used in the field of optical information communication, and relates specifically to a manufacturing method and a manufacturing apparatus for an optical fiber grating.
2. Description of the Related Art
An optical fiber grating is an optical element with the characteristic of attenuating or reflecting light of a specific wavelength.
Known types of optical fiber gratings include ultraviolet light induced optical fiber gratings (abbreviated to UV induced optical fiber grating below), for example. A UV induced optical fiber grating makes use of a phenomenon whereby irradiating ultraviolet light of a specific wavelength in the vicinity of 240 nm onto silica glass doped with germanium (abbreviated to germanium doped silica glass below) causes the refractive index thereof to rise, and is conventionally manufactured by the following steps, for example.
Generally, an optical fiber in which the core is made of germanium doped silica glass and the cladding is made of silica glass is prepared. Recently however, the optical fiber gratings are sometimes manufactured using optical fibers in which either the core and the cladding, or alternatively just the cladding, is made of germanium doped silica glass.
This optical fiber is then placed in a hydrogen atmosphere according to need, and hydrogen gas immersion treatment is performed to increase the sensitivity of the refractive index fluctuation relative to ultraviolet light.
In addition, when ultraviolet light is irradiated from a single direction onto the side of the optical fiber along the length direction at a predetermined period, using known methods such as an interference exposure method, a phase mask method, an intensity mask method, or a method in which an operation of performing exposure directly using a focused beam is repeated (step-by-step method), then the refractive index of the exposed portions of the optical fiber rises, forming a grating section in which a plurality of high refractive index sections are arranged intermittently at a predetermined period, and the refractive index fluctuates periodically along the length direction of the optical fiber.
Subsequently, a dehydrogenation process is performed, and heat aging is also preferably performed, thereby obtaining an optical fiber grating. The heat aging is performed with an object of improving the long term stability of the optical fiber grating.
In a short period fiber grating (abbreviated to “SPFG” below) in which the period of refractive index variation of the grating section (referred to as the grating period below) is comparatively short, a so-called reflective characteristic is obtained whereby light of a specific wavelength traveling the core in the same direction as the direction of incidence is reflected and attenuated. On the other hand, in a long period fiber grating (abbreviated to “LPFG” below) in which the grating period is comparatively long, a so-called radiating characteristic is obtained whereby light of a specific wavelength traveling the core in the same direction as the direction of incidence is coupled to cladding modes traveling in the same direction, and thereby attenuated.
However, it is known that in conventional manufacturing methods there is an attendant deterioration in the polarization dependence of the insertion loss of the optical fiber grating, regardless of type. The polarization dependence of the insertion loss (referred to as PDL below) is the difference between the insertion losses of the two polarization components constituting the light which propagates the optical fiber, and is particularly pronounced in optical fiber gratings with high transmission loss or high reflectance.
PDL is described below taking an LPFG as an example. This is because the optical characteristics of an LPFG are more sensitive than those of an SPFG to the characteristics of the optical fiber or the grating, specifically the anisotropy and birefringence, and the effects of any improvements are therefore more noticeable, although the same situation can be said to also apply to an SPFG.
The relationship in equation (1) holds true between the center wavelength λctr of the transmission loss of the LPFG (referred to as the “center wavelength” below) and the grating period Λ.λctr=Λ(ne1−nen)  (1)
Here, ne1 and nen refer to the effective refractive index of the guided mode (LP01) and the cladding mode (LP0n), respectively. When the optical fiber is birefringent, that is, ne1 and nen fluctuate due to polarization, this center wavelength λctr also fluctuates due to polarization as shown in equation (2), equation (3) and equation (4).
                              λ          ctr          MAX                =                  Λ          ⁡                      (                                          n                e1                MAX                            -                              n                en                MIN                                      )                                              (        2        )                                          λ          ctr          MIN                =                  Λ          ⁡                      (                                          n                e1                MIN                            -                              n                en                MAX                                      )                                              (        3        )                                          Δλ          ctr                =                                            λ              ctr              MAX                        -                          λ              ctr              MIN                                =                                    Λ              ⁡                              (                                                      B                    1                                    +                                      B                    n                                                  )                                      ≈                          Λ              ⁢                                                          ⁢                              B                1                                                                        (        4        )            
Here, B1 and Bn refer to the birefringence of the guided mode and the cladding mode, respectively. Here, attention is directed specifically to the refractive index of the guided mode.
The causes of this deterioration in PDL can be broadly divided into the two causes described below.
The first cause is polarization mode dispersion (referred to as PMD below) which occurs due to the difference between the effective refractive indices between polarization components. This is caused by slight ovality and eccentricity of the core of the optical fiber. The PDL caused by PMD becomes greater as the tilt of the transmission loss or the reflectance increases, but can be reduced to a certain extent by selecting an optical fiber with little ovality and eccentricity.
The second cause is non-uniform refractive index variation which occurs in the ultraviolet light exposure process.
FIG. 38A to FIG. 38D are diagrams describing the refractive index variation which occurs in a conventional ultraviolet light exposure process.
FIG. 38A is a perspective view showing a state in which ultraviolet light is irradiated from a single direction (the A direction) towards the side of an optical fiber 3 onto one location where the refractive index is to be raised.
FIG. 38B shows the refractive index variation in a cross-section of the optical fiber 3 caused by the irradiation intensity of this ultraviolet light, for the high refractive index section 3a formed in this manner. Because the intensity of the ultraviolet light increases towards the irradiation position of the ultraviolet light, there is a large rise in the refractive index, and a refractive index distribution develops in the cross-section of the optical fiber 3.
Here, the traveling direction of the optical fiber 3 is deemed the z axis direction, and the two directions which are orthogonal within the cross-section of the optical fiber 3 are deemed the x axis direction and the y axis direction.
It is known that the polarization state of the ultraviolet light irradiated onto the optical fiber 3 causes birefringence in the actual rise in the refractive index of the optical fiber 3. In other words, the rise in refractive index for a guided wave having an electric field with the same orientation as the electric field of the irradiated ultraviolet light is greater than the rise in refractive index for a guided wave having an electric field perpendicular in orientation to the electric field of the ultraviolet light.
As shown in FIG. 38C, the electric field of the ultraviolet light irradiated from the A direction can be considered to be divided into a y axis component and a z axis component. Of these components, the refractive index variation caused by the electric field of the y axis component presents birefringence in relation to the guided wave which is guided through the optical fiber 3. In other words, refractive index variation is greater for a guided wave having an electric field with an orientation in the y axis direction (called the Y polarization component for convenience) than for a guided wave having an electric field with an orientation in the x axis direction (called the X polarization component for convenience).
FIG. 38D is a diagram describing the anisotropy of the refractive index variation introduced at this time. The orientations of the polarization components which cause large refractive index variation are indicated by the bold arrows.
As a result, the difference in propagation constant between the X polarization component and the Y polarization component is large, and the PDL deteriorates. Because the refractive index variation caused by ultraviolet light having an electric field component in the z axis direction has an equivalent effect on the X polarization component and the Y polarization component, it does not need to be considered here.
FIG. 39 is a graph showing the refractive index variation for each polarization component in a case where the grating section is formed by irradiating ultraviolet light at a predetermined period along the length direction of the optical fiber 3 from the A direction only. It is apparent that there is a difference between the X polarization component and the Y polarization component in the amount of refractive index variation.
FIG. 40 is a graph showing an example of the optical characteristics of an optical fiber grating manufactured by this manufacturing method.
In this example, a cut-off shifted optical fiber (manufactured by Fujikura Co., Ltd.), for use with a band of 1.55 μm, in which the core is made of germanium doped silica glass and the cladding is made of silica glass, was used in the manufacture of a so-called radiative optical fiber grating with a grating period of 295 μm and a grating length (the length of the grating section) of 35 mm. Fine adjustment of the grating period was performed in the vicinity of 295 μm so that the wavelength in the transmission spectrum where the rejection ratio (transmission loss value) is the highest (referred to as the maximum rejection wavelength below) was 1530.0 nm. Furthermore, the ultraviolet light irradiation time and the power of the ultraviolet light were adjusted appropriately so that the transmission loss value at the maximum rejection wavelength was 4.0 dB.
A KrF excimer laser or an Ar-SHG (Argon-ion Second Harmonic Generation) laser or the like was used as the light source for irradiating ultraviolet light.
In the graphs, there are two peaks in the graph showing the PDL, but generally the highest peak is deemed the PDL worst case value. The PDL worst case value of the optical fiber grating of this example is 0.17 dB.
The other peak occurs due to the polarization of the ultraviolet light irradiated onto the optical fiber.
The state of the birefringence introduced as a result of the polarization of the ultraviolet light is shown in FIG. 41A and FIG. 41B. In these diagrams, the traveling direction of the light which propagates the optical fiber is deemed the z axis direction, and the two directions which are orthogonal within the cross-section of the optical fiber are deemed the x axis direction and the y axis direction.
It is reported in OFS-11, We 5-1 (1996), (T. Meyer, et al.) that the polarization state of the ultraviolet light irradiated onto the optical fiber affects the birefringence of the refractive index variation of the optical fiber. In other words, the rise in refractive index for a guided wave having an electric field with the same orientation as the electric field of the irradiated ultraviolet light is greater than the rise of the refractive index for a guided wave having an electric field perpendicular in orientation to the electric field of the ultraviolet light.
Here, as shown in FIG. 41A, the electric field of the irradiated ultraviolet light can be considered to be divided into a component which is parallel to the axis of the optical fiber, and a component which is perpendicular to the axis of the optical fiber. Because the refractive index variation caused by the component which is parallel to the axis of the optical fiber is axisymmetric, it is not a cause of the difference in the effective refractive index variation due to the guided wave, that is, it is not a cause of birefringence. However, regarding the perpendicular component, as shown in FIG. 41B, when exposure is performed from the x axis direction, a guided wave which has an electric field component oriented in the y axis direction has a higher refractive index than a guided wave which has an electric field component oriented in the x axis direction.
As described above, birefringence caused by ultraviolet light irradiation can be considered to be divided into the two types mentioned above, but in each case, a difference occurs in the size of the refractive index due to polarization.
This difference in refractive index due to polarization is shown in FIG. 42. As shown in FIG. 42, if the refractive index for the polarization B is higher than that for the polarization A, for example, then the disparity in the average refractive index of the grating section (abbreviated to the “DC component” below) is a cause of deviation in the center wavelength, and the disparity in the refractive index variation amount (abbreviated to the “AC component” below) is a cause of fluctuation in the maximum loss difference (rejection ratio). Both these factors are causes of PDL, and are particularly pronounced when the transmission loss and the reflectance of the optical fiber grating are high.
When actually manufacturing an LPFG, because the respective orientations of the birefringence caused by each of the two factors described above, that is the birefringence caused by the makeup of the optical fiber itself and the birefringence caused by exposure, are random, and these two types of birefringence can be added to each other or cancel each other out, it is assumed that even LPFGs manufactured by performing exposure under identical conditions can have complicated PDL characteristics.
The optical characteristics of an LPFG in a case where the respective orientations of the birefringence caused by the makeup of the optical fiber itself and the birefringence caused by exposure are taken into consideration is examined below.
A uniform LPFG transmission loss spectrum can be approximated closely by the sinc2 function shown in equation (5) below.
                              loss          ⁡                      (            λ            )                          =                              Δ            ⁢                                                  ⁢                          L              ·              sin                        ⁢                                                  ⁢                                          c                2                            ⁡                              (                                  π                  ⁢                                                            λ                      -                                              λ                        ⁢                                                                                                  ⁢                        ctr                                                              σ                                                  )                                              +                      L            ex                                              (        5        )            
The transmission loss spectrum of the LPFG is shown in FIG. 43. Here, λctr is the center wavelength of the transmission loss, σ is the bandwidth half-width, ΔL is the maximum loss difference, and Lex is the excess loss. Assuming a linearly polarized light for the sake of simplification, it is natural to assume that the period of the fluctuation of the center wavelength λctr and the maximum loss difference ΔL relative to the polarization direction of the incident light is 180°, and this can be approximated as shown in equations (6) and (7).
                              λ          ⁢                                          ⁢          ctr                →                              λ            ctr            0                    +                      Δ            ⁢                                                  ⁢                          λ              fib                        ⁢                                          cos                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                θ                            2                                +                      Δ            ⁢                                                  ⁢                          λ              exp                        ⁢                                          cos                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                φ                            2                                                          (        6        )                                          Δ          ⁢                                          ⁢          L                →                  Δ          ⁢                                          ⁢                                    L              0                        ⁡                          (                              1                +                                  ɛ                  ⁢                                                            cos                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      φ                                        2                                                              )                                                          (        7        )            
Here, Δλfib indicates the fluctuation width in the center wavelength caused by the birefringence of the optical fiber itself, Δλexp indicates the fluctuation width in the center wavelength caused by the DC component of the birefringence introduced as a result of exposure, and ε indicates the size of the fluctuation caused by the AC component of the birefringence introduced as a result of exposure.
The angle formed between the primary axis of the birefringence of the optical fiber itself and the primary axis of the birefringence introduced as a result of the exposure is defined as φ. In this case, it can be assumed that φ=θ+φ, and the transmission loss for a specific polarization with an angle of polarization of θ can be expressed as in equation (8).
                              loss          ⁡                      (            λ            )                          =                              Δ            ⁢                                                  ⁢                          L              ⁡                              (                                  1                  +                                      ɛ                    ⁢                                                                  cos                        ⁢                                                                                                  ⁢                        2                        ⁢                                                  (                                                      θ                            +                                                                                                                  ⁢                            ϕ                                                    )                                                                    2                                                                      )                                      ⁢            sin            ⁢                                                  ⁢                                          c                2                            ⁡                              (                                  π                  ⁢                                                                                    λ                        -                                                  (                                                                                    λ                              ctr                              0                                                        +                                                          Δ                              ⁢                                                                                                                          ⁢                                                              λ                                fib                                                            ⁢                                                                                                cos                                  ⁢                                                                                                                                          ⁢                                  2                                  ⁢                                                                                                                                          ⁢                                  θ                                                                2                                                                                      +                                                          Δ                              ⁢                                                                                                                          ⁢                                                              λ                                exp                                                            ⁢                                                                                                                                                                                                        ⁢                                                                      cos                                    ⁢                                                                                                                                                  ⁢                                    2                                    ⁢                                                                          (                                                                              θ                                        +                                                                                                                                                                  ⁢                                        ϕ                                                                            )                                                                                                                                      2                                                                                                              )                                                                    ⁢                                                                                                            σ                                                  )                                              +                      L            ex                                              (        8        )            
PDL is the difference between the maximum value and the minimum value of this loss (λ) when θ is varied from 0° through 180°, and can be expressed as in equation (9).PDL(λ)=loss (λ)MAX−loss (λ)MIN  (9)
From the above it is evident that, generally, PDL deteriorates in cases where the amount of fluctuation in λctr and ΔL are large, that is, in cases in which the birefringence of the optical fiber is large, and cases in which the birefringence introduced by exposure with ultraviolet radiation is large.
In order to solve this problem of the deterioration of PDL, a method described below is proposed in Optics Letters V. 19, n. 16, pp. 1260–1262 (Aug. 15, 1994).
FIG. 44A to FIG. 44D are explanatory diagrams showing this method, which differs from the method shown in FIG. 38A to FIG. 38D in that, as shown in FIG. 44, in addition to ultraviolet light being irradiated onto the side of the optical fiber 3 from one direction (the A direction), ultraviolet light is also irradiated from a direction (the B direction) which opposes this A direction. As a result, as shown in FIG. 44B, it is possible to solve the problem of bias in the refractive index in the cross-section of the optical fiber 3.
However, even in this method, as shown in FIG. 44C, since the ultraviolet light irradiated from the A direction is polarized in the y axis direction and the z axis direction, and the ultraviolet light irradiated from the B direction is also polarized in the y axis direction and the z axis direction, then the refractive index variation of the Y polarization component is greater than the refractive index variation in the X polarization component. As a result, the birefringence caused by the polarization of the irradiated ultraviolet light is not eliminated by this method.
FIG. 44D is a diagram explaining the anisotropy of the refractive index variation introduced at this time. The orientations of the polarization components which cause large refractive index variations are indicated by the bold arrows.
FIG. 45 is a graph showing the optical characteristics of an optical fiber grating manufactured in the same manner as in the example above, but with the exception that ultraviolet light was irradiated from two directions, the A direction and the B direction. The PDL worst case value is approximately 0.12 dB, which is slightly lower than that shown in FIG. 14. However, this value is not considered to be small enough, and further improvement is required.
In Japanese Patent Application No. 2000-360905, the inventors of the present invention proposed an exposure method in which birefringence caused by exposure is minimized, by irradiating ultraviolet light from four directions which are symmetrical about the axis of the optical fiber. In this method, the birefringence introduced into the fiber by the exposure can be reduced to a minimum, but the PDL resulting from the birefringence caused by the optical fiber itself can only be solved by using an optical fiber with minimal birefringence, that is minimal PMD.
The reason for this is because in this exposure method, the angle φ formed between the orientation of the birefringence of the optical fiber itself in the equation (8), and the orientation of the birefringence introduced as a result of the exposure is treated as indeterminable, and in equation (10),Δλctr=Δλfib+∫Δλexp cos 2φdl=Λ(Bfib+Bexp∫cos 2φdl)  (10)Bfib and Bexp are set as the birefringence of the optical fiber and the birefringence caused by exposure, respectively, and the second term in the right parentheses is set to zero, meaning that this is an exposure method in which the birefringence of the optical fiber and the birefringence caused by the exposure are not linked.